Satellites are used in many aspects of modern life, including earth observation and reconnaissance, telecommunications, navigation (e.g., global positioning systems, or “GPS”), environmental measurements and monitoring and many other functions. A key advantage of satellites is that they remain in orbit due to their high velocity that creates an outward centripetal force equal to gravity's inward force. Therefore, once in orbit, they stay there typically for years or decades. Since the velocities are so high (e.g., 3-8 km/s, depending on altitude), atmospheric drag should be minimized and/or avoided, which means satellites typically operate outside virtually any trace of the particles that constitute the atmosphere. In addition to drag, atmospheric collisions with particles, even at trace concentrations, can overheat, damage or eventually destroy the satellite.
Satellites are therefore clearly differentiated from atmospheric flying (i.e., airborne) vehicles such as airplanes, unmanned aerial vehicles (UAVs), helicopters or balloons, in which the atmosphere supports lift and the vehicles operate at velocities typically between zero (i.e., hovering) to 1-3 times the speed of sound and at altitudes below about 35 km.
Satellite orbital heights are typically categorized in three broad segments: low earth orbit (LEO), medium earth orbit (MEO) and geostationary earth orbit (GEO). The general uses and characteristics of these orbits are shown in Table I and represent generally accepted usage of the terms LEO, MEO and GEO. Satellites can orbit at any altitude above the atmosphere, and the gaps in altitude shown in Table 1, such as between LEO and MEO, are also used, if less regularly. It is also common that satellites may orbit in eccentric, non-circular orbits, thereby passing through a range of altitudes in a given orbit.
TABLE ITypical characteristics of common orbits.Altitude,Velocity,Orbitkmkm/sExemplary UsesCommentsLEO 400-2,0006.9-7.8Earth observation,Random orbits, 3-10 Ysensing, ISS, telecomlifetime, space junk issue,constellationslittle radiationMEO15,000-20,0003.5GPS, GLONASS,Highest radiation (VanEarth observationAllen Belt), equatorial topolar orbitsGEO42,0003.1Sat TV, high BWCan remain above sametelecom, weatherspot on Earth, typicallysatellitesequatorial orbits
For most satellites, their useful lifetime is determined by multiple factors. For example, in the case of GEO satellites, small fluctuations in solar winds and earth's gravity require regular use of fuel to maintain the satellite's position and attitude. Once exhausted of fuel, a satellite is typically rendered useless and decommissioned. However, due to GEO height, such a satellite itself will stay in orbit virtually forever due to its altitude and near zero atmospheric drag. Due to their apparent stationary position as viewed from earth's surface, they are widely used for telecommunications and satellite TV. Their large distance from Earth limits their usefulness in telephone services (time delay) and in high-resolution imaging (distance). They encounter solar winds and cosmic radiation that force use of very specialized and expensive electronics to survive.
MEO satellites are in the mid-range, mostly similar to GEO satellites except that they do not appear stationary when viewed from earth's surface. Their most common usage is for satellite positioning services, such as GPS, and certain Earth observation missions for which their trade-off in altitude between GEO and LEO is beneficial. Due to the presence of the so-called Van Allen Belts, these satellites can suffer large amounts of radiation and therefore require very specialized and expensive electronics to survive.
LEO satellites, conversely, may be in a constant state of very slight atmospheric drag requiring either regular boost to their altitude (e.g. fuel burns of typically chemical engines) or an end-of-useful-life caused by reentry and burn up similar to a meteor entering the earth's atmosphere. As an example, the International Space Station (ISS), orbiting at about 425 km, loses approximately 2-4 km/month of altitude and requires regular fuel burns to ensure it stays in proper orbit. But the atmospheric drag is still very low and LEO satellites can remain in orbit for years without fuel burns.
This relatively long life is the source of so-called “space junk”, in which any orbiting device can potentially collide with a useful satellite, thereby damaging or destroying it and creating additional orbiting objects. It is a widely recognized issue that at some density of space junk, probabilities of collisions increase, eventually leading to a virtually unusable orbit. A beneficial element of the current invention is to provide satellite services without increasing the space junk issue and furthermore to enable a mechanism that will be “self-cleaning” in the chosen orbits of 180-350 km.
Due to various shielding effects, especially of earth's magnetic fields, LEO satellites encounter little radiation and therefore do not necessarily require specialized and expensive electronics to survive. An exception to this rule is the so-called South Atlantic Anomaly, or SAA, which is a region in which a higher density of energetic particles may be found, causing short term interruptions of some electronics. This effect can be mitigated by many known techniques, so does not present a large issue for LEO satellites.
In fact, continual improvement in system operation is realized since by lowering the operating altitude, system components (e.g. optics, electronics, synthetic aperture radar (SAR), required solar panel area, etc.) can be made smaller, which in turn reduces vehicle size and drag, thereby enabling an even lower operating altitude, and so-on. While it is desirable to be closer to earth's surface (or any celestial body's surface, say Mars), atmospheric density effectively sets a lower limit on orbital altitude; or forces expensive, heavy counteracting systems such as on the Gravity field and steady-state Ocean Circulation Explorer satellite (GOCE), discussed below. For bodies without an atmosphere, such as earth's moon, there is no lower limit other than hitting the body itself.